Variable focal length lens system and image pickup unit

Abstract

A variable focal length lens system includes: first to fourth lens groups alternately having negative and positive refractive power; and an aperture stop; the first to fourth lens group being arranged to allow a space between the first and second lens groups to be decreased and spaces between the second and third lens groups and between the third and fourth lens groups to be varied, the third lens group travelling to allow image-plane position variation caused by subject position variation to be compensated and satisfying a following conditional expression, 0.05< Da/R 3 a <0.5  (1) where Da is a distance from the aperture stop to a most image-sided lens surface in the third lens group in the wide end state, and R3 a is a curvature radius of the most-image sided lens surface in the third lens group.

What is claimed is: 
     
       1. A variable focal length lens system, comprising:
 a first lens group having negative refractive power; 
 a second lens group having positive refractive power, wherein the second lens group is configured of a cemented lens comprising a first negative lens and a first positive lens; 
 a third lens group having negative refractive power; 
 a fourth lens group having positive refractive power; and 
 an aperture stop arranged between the first lens group and an image plane, 
 wherein a biconvex-shaped positive lens is arranged at a most-object-sided position in the fourth lens group, 
 wherein the first lens group to the fourth lens groups are arranged in recited order from an object plane towards the image plane, 
 wherein the first lens group to the fourth lens groups are configured to travel to allow a first space between the first lens group and the second lens group to be decreased, to allow a second space between the second lens group and the third lens group to be varied, and to allow a third space between the third lens group and the fourth lens group to be varied, thereby transition of the variable focal length lens system from a wide end state to a telephoto end state is accomplished, and 
 wherein the third lens group is configured to travel to allow image-plane position variation caused by subject position variation to be compensated and satisfy a following conditional expression,
   0.05 <Da/R 3 a< 0.5  (1)
 
 
 where Da is a distance from the aperture stop to a most image-sided lens surface in the third lens group in the wide end state, and 
 where R 3   a  is a curvature radius of the most image-sided lens surface in the third lens group. 
 
     
     
       2. The variable focal length lens system according to  claim 1 , wherein a following conditional expression is satisfied,
   0.25 <fw/|f 1|<0.7  (2)
 
 where fw is a total focal length of the variable focal length lens system in the wide end state, and 
 where f 1  is a focal length of the first lens group. 
 
     
     
       3. The variable focal length lens system according to  claim 1 , wherein the second and fourth lens groups travel together with each other in the transition of the variable focal length lens system from the wide end state to the telephoto end state. 
     
     
       4. The variable focal length lens system according to  claim 1 , wherein a following condition expression is satisfied,
   −1.3 <R 4 b/Db<− 0.4  (4)
 
 where R 4   b  is a curvature radius of an image-sided lens surface of the biconvex positive lens, and 
 Db is a distance from the aperture stop to the image-sided lens surface of the biconvex positive lens in the wide end state. 
 
     
     
       5. The variable focal length lens system according to  claim 1 , wherein the third lens group is configured of a single lens block. 
     
     
       6. The variable focal length lens system according to  claim 1 , wherein the first lens group is configured of three lenses comprising a meniscus-shaped negative lens having a concave surface facing towards the image plane, a second negative lens having a concave surface facing towards the image plane, and a second positive lens having a convex surface facing the object plane, the three lenses being arranged in recited order from the object plane toward the image plane. 
     
     
       7. An image pickup unit comprising:
 a variable focal length lens system configured to obtain an optical image; and 
 an image pickup device configured to output an image pickup signal based on the optical image, 
 wherein the variable focal length lens system comprises: 
 a first lens group having negative refractive power; 
 a second lens group having positive refractive power, wherein the second lens group is configured of a cemented lens comprising a negative lens and a positive lens; 
 a third lens group having negative refractive power; 
 a fourth lens group having positive refractive power; and 
 an aperture stop arranged between the first lens group and an image plane, 
 wherein a biconvex-shaped positive lens is arranged at a most-object-sided position in the fourth lens group, 
 wherein the first lens group to the fourth lens groups are arranged in recited order from an object plane towards the image plane, 
 wherein the first lens group to the fourth lens groups are configured to travel to allow a first space between the first lens group and the second lens group to be decreased, to allow a second space between the second lens group and the third lens group to be varied, and to allow a third space between the third lens group and the fourth lens group to be varied, thereby transition of the variable focal length lens system from a wide end state to a telephoto end state is accomplished, and 
 wherein the third lens group is configured to travel to allow image-plane position variation caused by subject position variation to be compensated and satisfy a following conditional expression,
   0.05 <Da/R 3 a< 0.5  (1)
 
 
 where Da is a distance from the aperture stop to a most image-sided lens surface in the third lens group in the wide end state, and 
 where R 3   a  is a curvature radius of the most image-sided lens surface in the third lens group. 
 
     
     
       8. The image pickup unit according to  claim 7 , wherein a following conditional expression is satisfied,
   0.25 <fw/|f 1|<0.7  (2)
 
 where fw is a total focal length of the variable focal length lens system in the wide end state, and 
 where f 1  is a focal length of the first lens group. 
 
     
     
       9. The image pickup unit according to  claim 7 , wherein the third lens group is configured of a single lens block. 
     
     
       10. The variable focal length lens system according to  claim 1 , wherein a following conditional expression is satisfied,
   2 <Ds/Y max<3  (3)
 
 where Ds is a distance from the aperture stop to the image plane in the wide end state, and 
 where Ymax is a maximum image height.